The present invention relates to a brush for a brush seal having a plurality of definably arranged ceramic fibers.
Brush seals are counted among non-hermetic types of construction, because as a rule, they operate under a certain leakage, in cases where different pressures prevail on each side of the seal. Because of the elastic yieldingness of the bristles, they are particularly non-sensitive to positional deviations of the component parts from one another, e.g. as to eccentricity, and operate with little wear and low friction. An important field of application of brush seals is in rotor-stator sealing locations in turbo machines. It is logical that the properties of a brush seal depend to a great extent on the properties of the bristles, and here, aside from the bristle geometry, attention must be paid primarily to the brush material. Criteria such as durable elasticity, wear resistance, temperature and media resistance, fracture resistance, etc, play a role. The use of bristles made of plastic, metal or ceramics is known, and possibly also of combinations of these. In using ceramic bristles, one hopes especially to attain high media and temperature stability, high wear resistance and the utmost durable elasticity. However, in view of their stiffness and brittleness, problems also arise. Ceramic bristles/fibers tend to be less elastically and not at all plastically deformable. They must also not be bent around tight radii, i.e. snapped over, since there is a danger of fracture. A further problem is the restraint, or rather fixing of the bristles/fibers in the position and distribution required for forming the brush. The customary joining methods for metals, like welding and soldering, are not applicable or applicable only with great limitations when it comes to purely ceramic fibers. It is possible to metallize the surface of ceramic fibers, so that a better wettability to fusible metals such as solder is attained. Metallizing fibers is costly, however, and besides that, the fiber""s properties are changed.
A method together with a device for producing brush seals is known from German Patent No. 196 41 374 C1, in which bristles cut to length are set in a definable position and distribution into one or between two brush end pieces and integrally connected as one material with one another and the brush end pieces. Ceramic bristles can also be processed in this manner, wherein the integral connection can be achieved, for instance, by using a suitable adhesive. Adhesives, such as those based on synthetic resin, often exhibit unsatisfactory media and temperature stability, and some of them tend to exhibit creep or shrinking. Ceramic, cementlike adhesives often exhibit insufficient adhesion, and tend to split off or form tears all the way to disintegration.
Among ceramic fibers, silicon carbide fibers (SiC fibers) are of special interest from the point of view of their properties. Trials have been made to combine SiC fibers with glass solders. But the fiber properties deteriorated so much by brittleness that this method of combination was not further pursued.
Brush seals are also known in which the bristles/fibers are held by friction locking, i.e. by friction under the effect of clamping or elastic forces. With hard, low-resilient and low friction ceramic fibers, strong forces and exact production techniques would be required for this, and the danger of snapping or breaking would again exist. Loosening of a few fibers could have the disintegration/destruction of the whole brush as a consequence.
In another technical field, namely the production of high-tensile fiber combination component parts, in the case of high temperature applications, so-called CMC""s (Carbon Matrix Composites) are known. As a rule, these are high-tensile carbon fiber structures (rovings, webs, knit fabrics, etc.) in a carbon matrix, i.e. they are carbon/carbon combinations (C/C, carbon/carbon). The carbon matrix can be produced, at least in part, by pyrolysis of a precursor resin, i.e. of a suitable, carbon-containing synthetic resin (carbon precursor).
On this matter, see European Patent No. EP 0 598 631 B1. In this document one can see that crystalline, powder-form SiC should be integrated into the structure, distributed space-wise, to be sure, as oxidation protection for the carbon structure.
Starting from the described related art, an object of the present invention is to provide a brush for a brush seal which stands out as an especially durable, inherently stable, media and temperature resistant and fiber-protective matrix for the ceramic bristles/fibers.
It has been shown that a carbon matrix produced by pyrolysis from a precursor resin, at great stability and durability makes possible the protective embedding of ceramic fibers, susceptible to snapping and denting, in view of high mechanical loading and durability of the brush, and therefore the brush seal as a whole.